Discussion on Standard Color Image of Prepress Digital Information
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Foreword
For today's prepress digital information, standard digital image archives are stored or opened using a variety of different specifications, and can be transferred from one software to another or from one platform to another. It is already a necessary trend.
These images can provide objective evaluation methods in image processing time, system performance, and image quality in the face of color output methods such as printing, proofing, faxing, etc., and can also encode images and data. The compression ratio, or data transmission efficiency is evaluated; in addition, the characterization can be performed on prints that have been subjected to conventional printing processes or digital direct output. The color reproduction system's color reproduction ability is good or bad, there are two ways to judge. The first is to make a subjective evaluation of the final output image (like color); the second is to make an objective assessment of the measurement results of the color measurement instrument.
Standard color images can be roughly classified into two types: natural image (natural image) and unnatural image (synthetic image). Natural images include flesh tones, high-bright colors, dark colors, neutral colors, hard-to-replicate trees, memory colors, complex geometric shapes, etc. image. Unnatural images include a resolution chart, a color chart, arabesques drawn from primary and secondary colors, and CMYK four primary colors.
All standard color images are divided into two digital encoding methods. In the first encoding method, the resolution of the image is 16 pixels per millimeter, the encoded value is 28 for 0% of the printed value (black), and 228 is 100% of the printed value (white). In the second encoding method, the resolution of the image is 12 pixels per millimeter, the encoded value is 0 for 0% of the printed value (black), and 255 is 100% of the printed value (white).
After the above encoding process, the obtained digital information can be accessed in the CD-ROM, and the file format is TIFF / IT file (ISO 12639).
Standard color image features
For the use of standard color images, there are several features:
(1) The amount of data can be confirmed
The smallest unit of digital image is bit (bit). By adding the total number of bits, you can confirm whether an image is completely converted or restored before and after image processing.
(2) Color sequence
Any color point in the image, whether it is to be printed or stored in a data file, can be controlled.
(3) Color values
That is, each pixel in the image is assigned a specific position and color value, which is represented by a Numeric method. The usual setting is 8 bits per unit pixel, that is, 256 colors. For quantized values, you can define their range.
(4) Network coverage ratio
The ratio of the area covered by the dot in the image, ranging from 0% to 100%. It is worth noting that the brightest part of the image has the smallest coverage of the network, which is close to or equal to 0%. The darkest part of the image has the largest coverage, close to or equal to 100%.
(5) The overall color change of the image
The relationship between the color value of the selected area of the image and the color value of the rest of the image. That is, the digital image can be modified separately for the color of a specific area in the image.
(6) Gray balance
In color reproduction, if the three primary colors reach a balance, the neutral gray can be achieved. Neutral gray produces a color cast that is easily noticeable because the human eye is particularly sensitive to gray. When the image is processed, if the neutral color of the neutral color and the copy are the same, then the hue balance of the color reproduction can be considered correct.
(7) Color Scanner (Color Signal Source)
A color scanner that reads photos into electronic digital signals through the reflection or transmission of light on the photo. These digital signals and the spatial domain of the overall image (spatial areas) related.
(8) reading direction
The content of the standard color image is right-reading when it is read. the way.
(9) Pixels
Pixel is a combination of the two letters Picture and Element, which is a unit for calculating a digital image. Like the photos of photography, digital images also have a continuous tone of light; if you zoom in on the image several times, you will find that these continuous tones are actually composed of many small squares with similar colors. These small squares are the images that make up the image. The smallest unit "pixel".
The image resolution is expressed in pixels per inch (ppi). When an image is printed or output, the number of dots and resolution of the image determine the height and width that appear on the printed document. Therefore, for images of the same point size, the higher the resolution, the smaller the printed image will be.
(10) In the pixel, the order of the four colors of cyan, magenta, yellow and black
In the image, the combination of the cyan, magenta, yellow, and black inks of each pixel is arranged in the same order as the next adjacent pixel.
(11) threshold value (threshold)
The grayscale division threshold of the image is to group the image according to its gray level. The general grayscale division divides the image into two gray values.
Any image that requires text recognition or streak recognition can be used to simplify a complex surface, so how to choose the appropriate and correct threshold value is an important task.
Description and definition of standard color images
Image information is divided into two types of digital information encoding:
The first, and the main coding method, has a range of information values ranging from 28 to 228, corresponding to 0% and 100% dot coverage. The information was sampled at a frequency of 16 pixels/mm (406 pixels/in) in a 128mm x 160mm image.
The second, also secondary coding method, has a value range of 0 to 255, corresponding to 0% and 100% of the dot coverage. The information was sampled at a frequency of 12 pixels/mm (305 pixels/in) in a 128mm x 160mm image.
Through the above two standard color image coding methods, the digital images produced by the computer each have a natural image (phto-graphed) and unnatural (synthetic) image.
ISO12640 uses the first image encoding method to produce eight natural images, the code name is N1~N8; and the natural image generated by the second image encoding method adds "A letter" to the N1~N8 code. That is, N1A~N8A. Unnatural images include resolution test charts and color tickets. The unnatural image produced by the first image coding method is coded as S1~S10; and the unnatural image generated by the second image coding method is added with the "A letter" in the code name, that is, S1A~S10A.
Natural images contain the following features:
(1) Image size
The first encoding method: 2560 pixels (length) X 2048 pixels (width)
The second encoding method: 1920 pixels (length) X 1536 pixels (width)
(2) pixel color composition
In the image, the combination of the cyan, magenta, yellow, and black inks of each pixel is arranged in the same order as the next adjacent pixel.
(3) color sequence
Cyan, Magenta, Yellow, and Black(Black).
(4) Color values
The first coding method: 8-bit binary signal, linear matching of dot coverage percentage. When the digital signal is 28, the dot coverage percentage is 0%. When the digital signal is 228, the dot coverage percentage is 100%.
The second encoding method: 8-bit binary signal, linear matching of dot coverage percentage. When the digital signal is 0, the dot coverage percentage is 0%. When the digital signal is 255, the dot coverage percentage is 100%.
(5) Image reading direction
When the image signal is read, its reading direction starts from the upper left corner of the image and ends in the lower right corner.
Unnatural images contain the following features
Unnatural image, resolution chart It is used to evaluate the analytical power, the cyber flower phenomenon, and various color effects that can be achieved with video output devices. Color chart
It is a layer application that compares and corrects colors.
Standard color images are stored in CD-ROM format ( Electronic data )
CD-ROM data format
The data stored on the CD-ROM contains 36 image file formats. The file name is taken according to the image name. Table 2 shows the file name, size, length and width, and related image names.
The TIFF (Tagged Image File Format) file format was developed by Microsoft and some other companies. It can be said to be the standard for digital image formats in the industry. It is flexible, expandable, portable and addable. Therefore, more and more manufacturers related to image processing adopt the TIFF file format, and more related tools are available. However, the TIF file is not without its shortcomings. Due to its inclusive features, it is inherently complex and difficult to manage or program than other file formats.
Conclusion
The pre-press operating system has been greatly improved, and the use of images and color images has increased dramatically. Coupled with the use of electronic and digital file exchange, the development of image and color management systems has grown at an alarming rate, which has also led to the trend of the pre-press process. Therefore, understanding the specifications of digital images is even more important.
Prepress personnel should evaluate the quality of digital images. They can compare the original images with the measurement basis. By changing the distribution of pixels, the quality of the scanned images can often be improved even beyond the quality of the originals. The quality of digital images is a result of a number of factors, including the quality of the original scan image, the resolution setting of the scan, the hard device of the scan, the skill of the operator, the resolution of the display, the gray balance, The dot area ratio, resolution, tone, natural color performance and other factors. Therefore, appropriate image quality should take into account factors such as user needs, system requirements, network infrastructure, and price.
The evolution of digital information will have a major change in the professional training of prepress workers. How to manage and store digital image files, open color management systems, and network transmission applications, etc., all have to follow the trend to be able to respond to the future.